In twin screw compressors there is a nominal maximum of 360xc2x0 of the helical groove or flute volume of each rotor that is trapped and compressed by the coacting rotors. Because the bores for the rotors overlap, the actual maximum helical groove or flute volume attainable for each rotor is more on the order of 330xc2x0. The overlapping bores create cusps in the nature of the waist of a figure eight. One of the cusps is the normal location for an unloader which moves axially in the cusp to unload and to control the Vi, or discharge pressure to suction pressure ratio, of the compressor. In one conventional unloading scheme, the unloader is normally exposed to both suction pressure and discharge pressure and as is acted upon by the difference in pressure which is driven by the solenoids. The axial movement of the unloader permits the use of axial porting which generally permits a larger port area and greater efficiency.
In the case of a tri-rotor, screw machine, the sun rotor has about 150xc2x0 of helical flute volume compression with each of the coacting rotors and about 30xc2x0 of overlap with each coacting rotor. The sun rotor is larger than the driven rotors such that the cusps formed by overlapping bores are asymmetrical. Forces tend to be large and require a large driving mechanism. This coupled with the short rotor length of the compression process make conventional unloader structure complicated and difficult to use.
The slide valves move in a plane perpendicular to the axis of rotation of the sun rotor and coact only with the sun rotor. The slide valves are located in the compressor discharge end bearing case such that the slide valve bodies are part of the end surface of the discharge end bearing case facing and sealing with the rotors. In the fully loaded position, the valve edge on the high pressure side of the slider of each slide valve defines part of an outlet port and is machined to the shape of the male, sun rotor profile and the desired full load Vi. The valve edge on the low pressure side of each slider is machined to a shape matching the male rotor profile or it may just be straight. When the slide valves are fully closed, the full load Vi is controlled by the male rotor axial porting which is controlled by the valve design of the high pressure side. The male and female rotor radial porting, as well as the female rotor axial porting, are designed to the same Vi which is designed to meet the lowest loading condition.
As a slide valve closes, it reduces the axial discharge port size and retards the start of compression by uncovering flutes thereby unloading refrigerant gas back to suction. The gas passing back to suction goes through the sun rotor which has a plurality of circumferentially spaced axial bores located radially inward of the root circle. The slide valve actuators are located in low pressure cavities such that only a small cross section of the slide valves see high pressure, therefore only a small force needs to be overcome by the slide valve actuators.
It is an object of this invention to provide a method and apparatus for unloading and Vi control for a multi-rotor screw compressor.
It is another object of this invention to provide a slide valve requiring a relatively low actuating force.
It is a further object of this invention to reduce the size of the actuator required and the sealing requirements in slide valves for a multi-rotor compressor. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically two slide valves are located in the discharge end bearing case of a multi-rotor screw machine and independently coact with the sun rotor of the multi-rotor screw machine for controlling capacity and Vi. Movement of the slide valves is in a plane perpendicular to the axis of the sun rotor.